Regardless of what my colleagues tell me, I have always liked the Modesto ash and the shamel ash as actually one of my favorite trees. I realize that ash innately exhibit inferior structural integrity and are susceptible to anthracnose, but they are just so rad. I was told of an epidemic of anthracnose during 1989 when I returned from Cal Poly in 1990, but I was not aware of how destructive the disease could be.

I was not initially alarmed by an unusually prolific infestation of anthracnose among some shamel ash I recently inspected in FremontCalifornia. Although the disease is usually present, it is not often so apparent among shamel ash. I then found it among two pair of Modesto ash in Willow Glen that had been nearly defoliated as quickly as the foliage emerged. When I found four more similarly afflicted specimens in Mountain View, I knew that this would be bad. I have since received many telephone calls and e-mail concerning the health of otherwise healthy ash, primarily Modesto ash, as well as the various species of sycamore.

This is unfortunately a ‘bad year’ for anthracnose because of the alternating rainy and warm weather with subsequent humidity that allows the proliferation of this fungus. Because the epidemic is so advanced and spores are very abundant, continued infestation is very likely if rain resumes or even next spring as dormant (secondary) spores become active. Infected Modesto ash should otherwise become foliated as warm and dry weather promotes foliar growth but inhibits fungal proliferation.

Fungal proliferation is inhibited during summer, but not eliminated, so foliage of any infected Modesto ash will likely be sufficiently infected by late summer to cause premature defoliation. Because the foliage will be necrotic at that time, it will not exhibit the typical yellow foliar color if it is still present in autumn. Shamel ash that are normally somewhat evergreen may now become unusually defoliated as older foliage is abscised when it would otherwise be replaced with new foliage, but the next phase of new foliage should help.

Sycamores are also affected by anthracnose, particularly the native California sycamores. The related ‘Yarwood’ London plane (sycamore) is less susceptible. Removal of foliar litter of any infected tree is recommended because it unquestionably contains viable dormant spores. Black foliar necrosis among rosicious species (of the family rosaceae) is another completely different epidemic of ‘fireblight’ that will be discussed later.